The present invention is related to applications assigned to the General Electric Co. and presently issued as U.S. Pat. No. 7,643,318 dated Jan. 5, 2010 and U.S. Pat. No. 7,643,319 dated Jan. 5, 2010 by Robert G. Wagoner.
The invention relates generally to gating signals for a three-phase, wye-connected H-bridge converter and more particularly to a zero-current notch waveform gating signal for minimizing harmonic distortion of the output waveform and increasing power output of the converter.
High-speed, high-power electric motors that operate at variable speed are increasingly required in a range of industrial, mining and drilling activities. Further, the activities often require a high-degree of reliability. In operations such as crude oil pumping from remote global locations where access to pumping stations is difficult and time-consuming, reliability of motor operation is necessary to prevent dangerous, costly and extended outages. Simple, sturdy and reliable power converters are requisites for such high-speed, high-power motor operations. It is well known that providing multiple individual components, such as series or parallel semiconductor switches, may increase the likelihood that any one individual component switch may randomly fail. Added elements such as snubber circuits for semiconductor switches, further increases the number of components that can fail. It is desirable to arrange the power converter in a simple configuration, with as low a part component count as is possible. However, individual components such as the semiconductor switches for the power converted must be operated with satisfactory margin to thermal and other functional limits to prevent failures in the simplified configuration.
A simplified three-phase, wye-connected H-bridge converter configuration is illustrated in FIG. 1. Each phase of the converter includes a power source/sink 20 with a dc power shaping circuit, represented by capacitor 30. The power source/sink/20 and dc power shaping circuit, represented by capacitor 30, establish a dc-link voltage input to the semiconductor switches of the bridge. Insulated-gate bipolar transistors (IGBTs) 40 with built-in diodes 45 may form each leg of the H-bridges 50, for example, but other power semiconductor switches such as integrated-gate commutated thyristors (IGCTs) or metal-oxide semiconductor field-effect transistors (MOSFETs) could be used instead. The type of power semiconductor switch is not important to the analysis. Each H-bridge includes two legs, an output leg 60 and a neutral leg 65. Each phase output, phase A 70, phase B 75 and phase C 80 is connected to the midpoint 85 of the respective output bridge leg 60. Each neutral connection to wye-point 90 is tied to the midpoint 95 of the respective neutral output leg 65. Output phase A 70, output phase B 75, and output phase C80 are connected to motor 90. The output or line current of the respective H-Bridge phase forms the respective load current Ia 91, Ib 92 and Ic 93 for the load motor 90.
Gating controls 35 provide control signals 36, 37, 38 for switching semiconductor switches 40 of Phases A, B, and C of the H-bridge converter, according to predetermined switching patterns. Gating controls may provide for synchronous switching or asynchronous (pulse-width modulation, for example) switching of the semiconductors switches 40 of the H-bridge.
However, to assure availability of operation of the motor loads, it is desirable to further reduce switching losses and harmonic distortion. Reduction in switching loss will keep semiconductor H-bridge switches operating at lower temperatures with a greater margin to failure. Accordingly, there is a need to provide a gating control scheme to further reduce switching loss and to minimize harmonic distortion.